Breast cancer cells must accumulate a series of genetic alterations before causing clinical disease. The mechanisms responsible for causing DNA damage in breast cancer are not known. LINE-1 (L1Hs) transposition is a mutagenic process that has not yet been investigated as a cause of breast cancer. The average human genome contains 100,000 L1Hs sequences; most are nonfunctional but 30-60 are potentially capable of transposition. Active elements amplify themselves by a process that involves an RNA intermediate and inserts a new L1Hs into the genome. When insertion occurs into a sensitive genetic locus the result is a new mutation. Examples of disease caused by L1Hs insertions have been described. We hypothesize that L1Hs retrotransposition is an important mechanism of mutagenesis in the development of either sporadic or hereditary breast cancer, or both. Several lines of evidence support this hypothesis. 1) A large fraction of hereditary breast cancers are the result of mutations in either the BRCA1 or BRCA2 genes. The BRCA1/2 proteins are involved in the repair of DNA double strand breaks (DSB). L1Hs transposition involves the formation of a DSB at the integration site; transposition rates may be enhanced in cells with altered pathways of DSB repair. 2) Trasposition by L1Hs requires the transcription and translation of L1Hs-encoded proteins. Differentiated cells, and most malignant cell types, do not transcribe or translate L1Hs. In contrast, p40, the protein encoded by the L1Hs first open reading frame, is present in most breast cancers and breast cancer-derived cell culture lines. This suggests that the controls that inhibit L1Hs transposition in normal cells have been deactivated in breast cancers. 3) Somatic L1Hs transposition in individuals with cancer, including a patient with breast cancer, have been reported, and retrotransposition has been observed in a breast cancer cell line. If L1Hs transposition is an important prelude to breast cancer, DNA isolated from patients' tumor cells will have L1Hs insertion sites that are not present in DNA isolated from the same patients' normal cells. These sites are called LINE Insertion Dimorphisms, or LIDs. We will use a new technique called L1 display to look for the presence of LIDs in paired blood and tumor DNA samples obtained from breast cancer patients. The relative frequency of LIDs in patients with sporadic and hereditary breast cancers will be compared with the frequency of LIDs in normal individuals. A significant difference in the frequency of LIDs in the study populations will suggest a role for L1Hs transposition in the development of breast cancer. It will also suggest that inhibiting the L1Hs encoded reverse transcriptase in high-risk patients may lower their incidence of breast cancer.